“The Last Sunset” is episode seventeen of season one of Space: 1999.
Victor and Commander Koenig are examining data on Planet Ariel and its solar system; Victor thinks it’s a perfectly balanced little solar system. Alan is taking two Eagles to Ariel when an alien object approaches. Alan is unable to avoid it and it latches onto his Eagle.
The commander orders them both to return to Alpha and the landing pad evacuated; he wants Alan to get as far away as possible after they land. The device hasn’t done anything yet; Koenig thinks it will wait until they are back. Victor is unable to scan the device.
The device still hasn’t done anything and is brought into the technical section. Koenig posits that it has been made to distract them from the civilisation that made it. Alan wants to head back to the planet. Victor still hasn’t discovered much about the device. Koenig asks if it’s inert. The professor tells him total. Just prior to it extending several probes and starting to vent gas.
They flee, Koenig ordering the technical section evacuated and sealed off and the airlocks opened. He wants Victor to analyse the gas. In Main Mission, David is asked how much the probe can contain. At its current rate, it could be infinite. Paul says the pressure was already way up before the airlocks were opened; the probe is emitting far more gas than it could have contained. More devices are coming in to land and Victor contacts Main Mission. The gas is air.
The sun rises to a blue sky. Most of the devices leave and Sandra says gravity is increasing. Dr Russell wants more data and tells the commander it looks good. He says the final test is to breathe the air. He asks for two volunteers to go outside and test it. He picks Sandra and Paul from everyone. They head outside in suits, gradually breathing the air, then removing their helmets.
People head outside to enjoy the fresh air and inside Koenig slides open a window. (A very poor design, sliding windows on a pressurised habitat, but deleted scenes had the commander ordering them to be fitted.) Paul and Sandra head away to talk. And kiss.
Victor is telling Koenig if they go into orbit, they can turn the Moon into a new Earth. Raise crops; they can make it rain. The commander says it took 25 years to reclaim the Sahara; there’s already a planet that supports some kind of civilisation. Their first priority is to see whether it can support them. At this point, the first device makes it rain. An odd coincidence.
Victor is enthused; the Sea of Tranquillity will become a sea and the craters will fill with water. Koenig points out Alpha is in one of those craters. He dispatches an Eagle with Dr Russell, Sandra, Paul and Alan to look for a new place to settle. The atmosphere is causing problems with communications, the Eagle runs into an electrical storm and loses contact, then crashes.
The atmosphere is breathable, but there’s not much else, and if they don’t orbit, the atmosphere will go. Koenig is sending Eagles to look for the one that crashed, but they’re having problems in the new atmosphere. Sandra was injured in the crash, they lost most of their supplies and then Paul starts acting oddly. Possibly he shouldn’t have tried the mushrooms he found.